Although often associated with dry low-NOx combustors used in some gas turbine engines, strong dynamic pressures due to combustion oscillations or pulsations have also occurred in diffusion-flame type combustors when operated on natural gas fuel. In the past, these pulsations have occurred during engine startup operation at or near full-speed, no-load conditions. These pulsations can reach potentially damaging amplitudes, but they remain at these levels for relatively short periods of time during engine startup.
These combustion oscillations have been attributed to fuel flow fluctuations that are induced by dynamic pressures under the low steady flows of fuel at no-load conditions, during which the fuel nozzles are acoustically “soft” (i.e., when the fuel nozzles have low acoustic resistance). These flow fluctuations, in turn, give rise to the burn-rate fluctuations that produce the associated pressure fluctuations.
These pressure pulsations are not limited to no-load conditions. For example, strong dynamic pressures at full-load conditions have occurred in turbine engines with diffusion-type combustors. Normally, pressure pulsations would not be expected under full-load operation because the full-load fuel flow rate is approximately five times greater than the no-load flow rate. Thus, the fuel nozzles would have a higher acoustic resistance at full-load than at no-load. However, experience has demonstrated that combustion oscillations can still occur at full-load.
There are strong indications that the fuel delivery system participates in the combustion dynamics under no-load conditions. It has been observed that these pulsations in the combustor and in the fuel feed line share a common excitation source; other analyses performed on this data suggest that this pressure pulsation source is the combustor. Further study reveals that, even under operating conditions where fuel flow fluctuations due to combustor dynamic pressures are expected to be suppressed, acoustic communication between the combustors and their feed lines is still strong. Thus, there is a need for the nozzle to be dynamically stiffened at the frequency of these fluctuations.